When lava flows through forested areas, you sometimes find holes like this:

Tree Mold

Hawai'i Volcanoes National Park

4 November 2006

1/180 @f/6.7; Pentax *ist DS; zoom lens at 18mm

When the lava hits trees, it begins to cool and harden almost immediately. The trees burn, but this can take a while, even in the middle of a lava flow. The end result is seen in the picture above - a tree shaped hole, known as a tree mold, that preserves the shape and occasionally even the surface texture of the tree.

They have this sort of thing in Lava Beds National Monument in Northern Northern California:http://www.nps.gov/labe/index.htm
This a cool place because these are lava tubes: you enter at one place and come out another. They have some tours, but you can just grab a flashlight and wander about on your own if you want. Try that at Carlsbad!